As the complexity of circuits grows, the need to increase the density of integrated circuits onto a PC (printed circuit) board increases a major advancement in the field of circuit board technology has been the advent of surface mount technology to attach and connect integrated circuit components onto printed circuit boards. As is known in the art, surface mountable integrated circuit packages allow connection of a package to a planar surface of a circuit board without the need for plated-through holes in the circuit board. Surface mount technology thus increases the theoretical component density of the circuit board, as well as the degrees of freedom available to the board designer, as the location of integrated circuit leads need only be considered for a single surface plane of a multi-layer circuit board, rather than for all planes of the board.
To meet the challenge of increased density of integrated circuits on a PC board, the BGA (ball grid array) integrated circuit package type has become popular in the field. Conventional BGA packages are similar in layout and arrangement to the PGA (pin grid array) packages, in providing an array of connections on the underside of the integrated circuitry packages. Instead of the pin connectors used in PGA packages, however, BGA packages utilize a solder ball located at each connector location. As is known in the art, the BGA package is attached to a printed circuit board by reflowing the solder balls to make connection to conductors at the surface of the printed circuit board. The BGA package provides the important advantage of being self-aligning, as the surface tension of the solder will tend to pull the BGA package into proper alignment with the corresponding conductors on the printed circuit board.
For background purposes, BGA packages of various types are known in the art, including those of "cavity-up" and "cavity-down" types. Conventional cavity-up BGA packages mount the integrated circuit chip face-up into a cavity (or onto the surface) of the package substrate, attach wire bonds between the package and the chip on this side, and then either transfer mold or otherwise dispense a plastic over the chip and bond wires to provide environmental protection to the chip and wires. The solder balls are provided on the side of the substrate opposite the chip, and the packaged chip is then mounted to the system printed circuit board. Conventional cavity-down integrated circuit packages mount the integrated circuit chip into the cavity of, or onto the surface of, the substrate, followed by attachment of bond wires thereto and molding of the plastic around the chip. In this type of package, the solder balls are provided on the same side of the substrate as the chip, so that the chip is disposed upside-down after attachment of the packaged chip to the circuit board.
A major disadvantage of using BGA packages is the amount of thermal and mechanical stress exerted on the package during the assembly procedure, relative to non-surface-mountable packages, such as dual in-line (DIP) and similar packages. The mounting of a dual-in-line integrated circuit package to a circuit board is accomplished by wave solder of the underside of the circuit board as the pins of the integrated circuit package extend through plated-through holes in the circuit board. Accordingly, the circuit board itself insulates the integrated circuit package body from the high solder temperatures and harsh chemicals to which the soldered lead tips are subjected. Since the leads of a BGA package are soldered at the same surface of the printed circuit board as the package, however, the BGA package and its contents are directly subjected to the high temperature, and to harsh chemicals used in the soldering process, such as flux, solder and cleaning solvents.
While the semiconductor device encapsulated within the BGA package are typically able to withstand the environmental stresses of the surface mounting process, other components which may be part of the package may not be sufficiently tolerant. In particular, the use of battery power for many electronic circuit functions has become available, primarily due to advances made in complementary metal-oxide-semiconductor (CMOS) fabrications and design technology. As is well known, CMOS integrated circuits are able to operate with extremely low active power requirements; in the case of CMOS memory devices, such as static random access memories (SRAMs), the power requirements for data retention are especially low. These low power requirements allow operation and data retention in electronic systems to be powered by conventional lithium batteries and other cell types, improving the portability and reliability of a modern system, without the need for a backup power supply on the system board itself. Unfortunately, conventional batteries are unable to reliably withstand the temperatures and chemical conditions to which an integrated circuit is subjected during surface mount assembly. Some types of batteries can be irreparably damaged by exposure to temperatures as low as 181 degrees Centigrade, which is below the temperature of certain solder used in the surface mount process. Other components, such as quartz crystal resonators used in connection with on-chip oscillators, are also vulnerable to these harsh environmental conditions.
In a "gull wing" surface mountable package, use of a removable module which attaches to the package has been proposed in U.S. application Ser. No. 08/114,750, entitled "Surface Mountable Integrated Circuit Package With Detachable Module" to Siegel et al, filed Aug. 31, 1993 (Attorney Docket No. 93-C-53) and U.S. application Ser. No. 08/225,227, entitled "Surface Mountable Integrated Circuit Package With Low-Profile Detachable Module", to Siegel et al, filed Apr. 8, 1994 (Attorney Docket No. 94-C-44), both assigned to SGS-Thomson Microelectronics, Inc. and incorporated by reference herein. BGA, however, typically have very little clearance above the system PC board to allow the use mechanical connecting means of the type described.
Therefore, a need has arisen in the industry for a BGA package which supports the use of components which cannot withstand the environmental stress of surface mounting processes.